1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly to an apparatus which is arranged to ensure the high quality of a video signal in such case that a mean value-interpolation is to be effected when a drop-out compensation is to be effected or when a frame video signal is formed from a field video signal.
2. Description of the Related Art
In reproducing a video signal recorded on a recording medium in a frequency modulated state, some of known video signal reproducing apparatuses have been arranged to compensate for drop-outs that occur in the course of reproduction or to carry out a mean value interpolation of frame the event of forming a video signal in unit from a video signal of field unit. The conventional apparatus of this kind has been arranged as shown in a block diagram in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, an input terminal 1 is arranged to receive a frequency modulated video signal which is reproduced from a recording medium. The apparatus comprises a switch 2; a one-horizontal scanning period (hereinafter referred to as 1 H period) delay line 3; and a drop-out detection circuit 4. The circuit 4 is arranged to produce a drop-out detection signal. The drop-out detection signal is at a high level when a drop-out takes place in the reproduced signal. The connecting position of the switch 2 is controlled by the drop-out detection signal. If a drop-out takes place, the position of the switch 2 is shifted to one side H thereof. By this, the reproduced video signal is compensated for the drop-out with the output of the 1 H period delay line 3.
Frequency demodulation circuits 6 and 7 are arranged to frequency demodulate the output of the 1 H period delay line 3 and that of the switch 2 respectively. Low-pass filters (LPFs) 8 and 9 are arranged to remove unnecessary frequency components from the frequency demodulated signals respectively. Each of the LPFs 8 and 9 has a delay time T1 between signal input time and output time. The apparatus includes low impedance converters 10 and 11; resistors 12 and 13; a switch 14; and an input terminal 15, which is arranged to receive a mean value interpolation gate pulse signal as will be further described later herein. The outputs of the low impedance converters 10 and 11 are supplied to an adding point of the resistors 12 and 13 which are of equal resistance values. The output of the adding point and the output signal of the low impedance converter 11 are supplied respectively to the two terminals of the switch 14. In case where the video signal reproduced from a recording medium (not shown) is a field video signal, in order to form a frame video signal in simple manner the switch 14 is so arranged that the change-over operation thereof is controlled at the field periods by a mean value interpolation gate pulse signal in coming from the input terminal 15. The mean value interpolation gate pulse signal consists of low and high level pulses which alternately appear at the field periods. The position of the switch 14 is on one side A thereof when the mean value interpolation gate pulse signal is at a low level and is on the other side B thereof when the gate pulse signal is at a high level. A mean value interpolation signal which is arranged to be taken out from the addition point of the resistors 12 and 13 is allowed to be produced while the switch 14 is on the side B. Under the control of the switch 14, however, the mean value interpolation signal taken out from the addition point is not allowed to be produced from the switch 14 at its equivalent pulse section where pulses of 1/2 H (1/2 of the horizontal scanning period) exist.
The apparatus further includes subtracters 16 and 18; a switch 17; a deemphasis circuit 19; and a reproduced video signal output terminal 20.
Referring to FIGS. 3(A) to 3(H), when drop-out pulses D1 and D2 which are as shown in FIG. 3(A) are produced from the drop-out detection circuit 4, the low impedance converter 11 produces a video signal which is as shown in FIG. 3(C). As apparent from the illustration, an inverting phenomenon is caused to take place in the reproduced video signal by the incontinuity of the reproduced frequency modulated video signal at the rising and falling points of the drop-out pulses D1 and D2. Then, depending on the time constant of the deemphasis circuit 19, this results in a tailing phenomenon, which is conspicuous to the visual sensation to deteriorate picture quality particularly in the event of reproduction of a still image. Therefore, it has been practiced to carry out a subtracting operation by means of the subtracter 16 on the output of the low impedance converter 10 which is the video signal output obtained by delaying it by one H period and the output of the low impedance converter 11 which is obtained without the delay of one H period. A non-correlating signal is thus obtained including the inverted pulse resulting from the incontinuity of the frequency modulated video signal. Then, the edge detection circuit 5 generates pulses of a pulse width T2 which is wider than T1. The level of these pulses becomes high at the rising and falling points of the drop-out pulses D1 and D2 and remains high during a period of time T2 (longer than T1) as shown in FIG. 3(B). The switch 17 is closed only for the period of time during which the pulses from the edge detection circuit 5 are at the high level. Then, the subtractor 18 subtracts from the reproduced video signal only the inverted pulse which is included in the noncorrelating signal produced from the above stated subtracter 16. After that, the reproduced video signal is produced via the deemphasis circuit 19 and the output terminal 20.
However, in cases where a frame video signal is to be obtained by mean value interpolation from a field video signal, a drop-out occurs during a horizontal scanning period of the reproduced, frequency modulated video signal within a field period during which the mean value interpolation is made as shown in FIGS. 3(D) and 3(E). With the drop-out occurred in this manner, if the inverted pulse PR takes place within the demodulated reproduced video signal, the inverted pulse PR also takes place in the mean value interpolation signal formed by using the portion of the reproduced video signal obtained during the horizontal scanning period having the inverted pulse PR. In that instance, the level of the inverted pulse PR attenuates to 1/2 of the level thereof obtained before the interpolation process. However, it has been impossible to completely remove the inverted pulse PR from the mean value interpolation signal formed by the conventional apparatus.